1. Technical Field
The present disclosure generally relates to the field of illumination devices and, more particularly, to control dusk-to-dawn illumination.
2. Description of the Related Art
Energy conservation has become of ever increasing importance. Efficient use of energy can result in a variety of benefits, including financial benefits such as cost savings and environmental benefits such as preservation of natural resources and reduction in “green house” (e.g., CO2) gas emissions.
Residential, commercial, and street lighting which illuminate interior and exterior spaces consume a significant amount of energy. Conventional lighting devices or luminaires exist in a broad range of designs, suitable for various uses. Lighting devices employ a variety of conventional light sources, for example incandescent lamps, florescent lamps such as high-intensity discharge (HID) lamps (e.g., mercury vapor lamps, high-pressure sodium lamps, metal halide lamps).
There appear to be two primary approaches to reducing energy consumption associated with lighting systems. One approach employs higher efficiency light sources. The other approach selectively provides light only when needed.
Use of higher efficiency light sources may, for instance, include replacing incandescent lamps with florescent lamps or even with solid-state light sources (e.g., light emitting diodes (LEDs), organic LEDs (OLEDs), polymer LEDs (PLEDs)) to increase energy efficiency. In some instances, these higher efficiency light sources may present a number of problems. For example, florescent light sources take a relatively long time after being turned ON to achieve their full rated level of output light or illumination. Such light sources also typically have a high energy consumption during warm-up. Many higher efficiency light sources emit light with a low color rendering index (CRI). For reference, sunlight has a CRI of 100 and represents “ideal light” which contains a continuous spectrum of visible radiation. Low CRI light is less pleasing to the human eye. Surfaces illuminate with low CRI light may not be perceived in their “true” color. Low CRI light makes it more difficult to discern details, often requiring a higher level of output light or illumination to discern details that would otherwise be discernable in high CRI light. Further, higher efficiency light sources may require additional circuitry (e.g., ballasts) and/or thermal management techniques (e.g., passive or active cooling).
Providing illumination only when needed can be achieved manually by a user of the lighting system, or automatically through the use of one or more control mechanisms. Automatic control mechanisms generally fall into two broad categories, timers and environmental sensors. Timer based control mechanisms turn light sources ON and OFF based on time. The times are typically user configurable. Such relies on the user to account for changes in length of daylight which may occur throughout a year. Very often, timer based control mechanisms are set once and never updated. Environmental sensor based control mechanisms sense light or illumination level and/or motion or proximity. Light or illumination level based control mechanisms are commonly referred to dusk-to-dawn sensors. Dusk-to-dawn light or illumination level based control mechanisms turn the light sources ON when a level of light or illumination in an environment falls below a turn ON threshold, and turn the light sources OFF when the level of light or illumination exceeds a turn OFF threshold. Light or illumination level based control mechanisms advantageously automatically accommodate changes in length of daylight throughout the year. However, such control mechanisms typically employ fixed turn ON and turn OFF thresholds (e.g., 10 Lux and 30 Lux, respectively). Since the particular application or location of the luminaire is not known, the manufacturer sets the turn ON and turn OFF thresholds to accommodate a wide range of conditions. Thus, the thresholds may be set too low for applications or locations with a high volume of artificial light. Likewise, thresholds may be set too high for applications or locations with no or relatively little artificial light. Such may cause the light source(s) to be turned ON prematurely or turned OFF later than would otherwise be possible, resulting in less efficiency than might otherwise be possible. Such may also cause the light source(s) to be turned ON late or turned OFF prematurely, failing to provide adequate light when needed. Even changes in natural ambient light may cause the thresholds to be incorrect. For example, snow may reflect sufficient light to prevent the light source(s) from being turned ON.
Motion or proximity based control mechanisms (e.g., passive infrared sensor based mechanisms) turn light sources ON when motion or proximity is detected. Motion or proximity based control mechanisms turn light sources OFF after some period of time if no motion or proximity is detected during that period of time. Sensitivity of such motion or proximity based control mechanisms is typically user configurable, as is the duration between turn ON and turn OFF. However, motion or proximity based control mechanisms have limited range (e.g., 10 meters), limiting the number of applications in which such may be effectively employed. Motion or proximity based control mechanisms may also be ineffective where the ambient air temperature or temperature of an object is close to that of the trigger temperature (e.g., temperature of human body). Some lighting control mechanisms employ both light or illumination level based and motion or proximity based techniques. Such lighting control mechanisms turn light sources ON only if motion is detected while the level of light or illumination in the environment is below the turn ON threshold. Thus, the motion or proximity sensing is active only between dusk and dawn.
In addition, a failure of the automatic control mechanism, for example failure of a photosensor used to turn the light source ON or OFF dependent upon the measured ambient light level may result in the light source remaining in a continuously ON state in the event the automatic control mechanism fails in a “closed” position permitting current flow to the light source or in a continuously OFF state in the event the automatic control mechanism fails in an “open” position interrupting current flow to the light source. Either failure mode results in an unacceptable mode of operation of the light source.
Sometimes these approaches are incompatible with each other. For example, the relatively “warm up” long time required for high intensity discharge light sources to produce full output hinders the effective use of such light sources with motion or proximity based control mechanisms. Typically, high intensity discharge light sources cannot be dimmed. While some dimmable compact florescent light sources have recently become available, such exhibit poor power factors. Further, many control mechanisms are built into the luminaire. Such makes it difficult or even impossible to modify operation of the control mechanism beyond some simple user settings (e.g., sensitivity, duration between turn ON and turn OFF).
New approaches to improving the performance of lighting systems in the event of automated control mechanism failure are therefore needed.